This invention relates to a hydraulic control arrangement for a roof support unit of a longwall mineral mining installation having a conveyor and a plurality of roof support units positioned side-by-side along the goaf side of the conveyor, and in particular to hydraulic control means for controlling the advance of roof bar extensions of the roof bars of the roof support units.
The roof bar extensions of such an installation are advanced towards the face being won by hydraulic rams associated with the corresponding roof bars. The roof bar extensions support the roof of the mineral mining working in the critical region adjacent to the face. It is important, therefore, to advance the roof bar extensions as soon as possible after the conveyor has been advanced following a winning run of the plough (or other winning machine) along the face side of the conveyor. Known systems for advancing the roof bar extensions incorporate either manual control means or automatic control means.
A known hydraulic control arrangement is effective to advance the roof bar extension of a roof support unit by the same distance as that through which the advance ram of that unit advances the associated conveyor section. The hydraulic ram which advances the roof bar extension is actuated by the movement of the advance ram which advances said conveyor section. The resulting synchronisation of the roof bar extension ram and the advance ram enables the roof of the working to be supported reliably, at all times, even in the critical region adjacent to the face. The roof bar extension ram is controlled by a metering ram having a metering chamber which communicates with the working chamber of the advance ram that diminishes in volume as the associated conveyor section is advanced. Thus, the hydraulic fluid displaced from this working chamber of the advance ram passes into the metering chamber. Then, by pressurising the metering piston of the metering ram, a metered amount of hydraulic fluid is forced out of the metering chamber into the appropriate working chamber of the roof bar extension ram, so that the roof bar extension is advanced towards the mineral face (see British Pat. No. 2 068 050).
A form of hydraulic control arrangement of this type has the metering ram formed within the advance ram. In this case, the metering chamber is formed within the piston rod (which is hollow) of the advance ram, the metering piston being guided in the hollow piston rod and being braced against the end wall of the cylinder. When the conveyor is advanced, the metering piston forces pressurised hydraulic fluid out of the metering chamber and into the appropriate working chamber of the roof bar extension ram, so that the roof bar extension is advanced towards the mineral face in synchronism with the advance of the conveyor. The metering chamber is arranged to have a cross-section which corresponds to that of said working chamber of the roof bar extension, so that the roof bar extension is advanced by the same distance as the conveyor (See DE-OS No. 3 217 822).
The aim of the invention is to provide an improved hydraulic control arrangement of this type. In particular, the hydraulic control arrangement should achieve reliable synchronous control without the need for matching the cross-sectional area of the roof bar extension ram absolutely to the cross-sectional area of the metering ram.